Synthesis gas (“syngas”) is substantially comprised of carbon monoxide and molecular hydrogen and is generally produced from natural gas, gasified coals, or other sources of light hydrocarbons (“light hydrocarbon gas” or “feed gas”). “Light hydrocarbon gas” includes hydrocarbon gasses composed primarily of hydrocarbons having a carbon number of less than or equal to 4. Syngas is used as a feedstock for Fischer-Tropsch synthesis wherein the syngas is converted into higher molecular weight hydrocarbons, including, for example, olefins, paraffins and alcohols. In a Fischer-Tropsch hydrocarbon synthesis reaction carried out at low or medium pressure, i.e. in the range of about atmospheric to 500 psig, over a cobalt catalyst the optimal H2:CO molar ratio in the syngas is about 2:1.
Three basic methods have been employed for producing synthesis gas. A first known method is steam reforming wherein one or more light hydrocarbons such as methane are reacted with steam over a reforming catalyst to form carbon monoxide and hydrogen. The steam reforming reaction is endothermic and a reforming catalyst containing nickel is often utilized.
A second known method is partial oxidation wherein one or more light hydrocarbons are combusted in the presence of a stoichiometric deficiency of oxygen to produce synthesis gas. The partial oxidation reaction is typically carried out using expensive high-purity oxygen and may proceed with or without a catalyst.
In a third known method, partial oxidation and steam reforming are combined in a process known as autothermal reforming (“ATR”), wherein air or enriched air instead of high-purity oxygen, may be used as a source of oxygen for the partial oxidation reaction. In the ATR process, the exothermic heat of partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. The process may be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby a cost advantage is achieved.
In conventional autothermal reactors a burner is frequently used to combust the light hydrocarbon stream with an amount of an oxidant, which may be air or oxygen-enriched air or pure oxygen. The combustion product is then passed through a reforming catalyst to convert the oxidation product into a synthesis gas at equilibrium conditions at the temperature and pressure in the autothermal reactor. A major problem with such conventional ATR reactors is the formation of soot in the high temperature region associated with the burner which represents wasted carbon and can constitute an undesirable plugging material in the catalyst bed. In order to prevent excessive soot formation, a relatively high amount of steam is used. However, higher steam levels, for example, steam to natural gas ratios in excess of 0.6:1, lead to reductions in the amount of CO produced and tend to increase the H2:CO product syngas ratio above the desired 2:1 molar ratio.
The ATR process typically results in a lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, steam reforming methods generally result in an H2:CO molar ratio of about 3:1 or higher. When the feed to the ATR process is a mixture of light shorter-chain hydrocarbons, such as a natural gas stream, some form of additional control is usually necessary to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at an optimum ratio of about 2:1. For this reason, steam and/or CO2 may be added to the synthesis gas reactor to adjust the H2:CO molar ratio of the syngas.
Some prior methods have employed a two-zone ATR reactor in which homogeneous combustion occurs in the first zone and reforming occurs within the second zone. However, the two-zone ATR burner typically involves a costly and complicated design to prevent mechanical degradation due to excessive temperatures. Moreover, in a two zone ATR system, the light hydrocarbon feed gas and oxygen must be completely premixed and injected within a very short residence time prior to ignition in order to prevent backlighting and/or oxidation combustion within the injection nozzle.
The burner injection system and the need for a homogeneous oxidation reaction often is a limiting factor in scale up of conventional ATR systems. Furthermore, partial reduction can lead to volatile compounds (e.g., suboxides), and subsequent mechanical degradation of the reactor walls. Additionally, the volatile suboxides may precipitate in cooler sections of the reactor downstream from the oxidation zone.
It would be desirable to provide an ATR reactor and method of producing synthesis gas which operates at a lower temperature, in an efficient manner, such that excessive amounts of soot are not produced. In particular, a reactor that can more efficiently convert a light hydrocarbon feed gas, such as natural gas, to synthesis gas without a flame and at lower temperatures would be highly desirable.